The present invention relates to an active matrix liquid crystal display in which the pixels formed by liquid crystal cells are arranged in rows and columns of a matrix. One side of the liquid crystal cells is connected to a reference potential and the other side is connected column-by-column to column electrodes via controllable switches assigned to them individually. The switches are connected row-by-row to row electrodes on the control side. The column electrodes are connected to a column control unit generating various gray-scale signals for the different columns and the row electrodes are connected to a row control unit generating turn-on signals for the switches in the various rows. The terms rows and columns are interchangeable here as well as hereinafter.
Such an active matrix liquid crystal display which is also referred to as a TFT-LCD (thin film transistor liquid crystal display) due to the controllable switches commonly in the form of thin film transistors, is described in U.S. Pat. No. 4,635,127.
To display images with various gray-scale values, gray-scale signals are applied to the column electrodes, the gray-scale signals representing the gray-scale values of one row each; the gray-scale signals are switched through to the liquid crystal cells of the affected row via a turn-on signal at one of the row electrodes. In this manner, all rows having liquid crystal cells are activated in rapid succession. The optical transparence of each individual liquid crystal cell is set as a function of the voltage at the liquid crystal cell so that the desired image is displayed when there is background lighting of the active matrix liquid crystal display. To avoid gray-scale value corruptions in image rendering, the liquid crystal cells are operated in a voltage range in which the otherwise typically non-linear dependence between the transparence of the liquid crystal cells and the voltage applied to them is approximately linear.
For the display of color images, red, green and blue color filter color strips are arranged upstream or downstream from the liquid crystals alternatingly column-by-column, the three adjacent liquid crystal cells lying upstream or downstream in a row being combined into one color pixel made up of three subpixels as regards their activation. In rendering color images, non-linearities between the transparence of the liquid crystal cells and the voltage applied to them can have a particularly interfering influence.
The transparence of each individual liquid crystal cell occurring as a function of the applied voltage depends on the viewing angle due to the voltage-dependent optical twisting of the liquid crystal, so that with a specific voltage applied to the liquid crystal cell, the brightness of the displayed pixel varies as a function of the viewing angle of the observer.
This effect effect is used in liquid crystal displays which are designed only for bright/dark or black/white display, but not for displaying different brightness and gray-scale values. An example of this is the setting of optimum contrast ratios for a specific viewing angle. An additional example described U.S. Pat. No. 5,526,065 is the use of such a liquid crystal display as an optical filter in front of a conventional screen in a vehicle to make the image displayed invisible for the driver""s viewing angle range during travel but visible to the front-seat passenger.
An object of the present invention is to increase the voltage range within which the liquid crystal cells of an active matrix liquid crystal display can be operated without corruption of the rendered image and in addition, to increase the application possibilities of such an active matrix liquid crystal display.
According to the present invention, this objective is attained in that the active matrix liquid crystal display of the aforementioned type has a correction device which distorts the gray-scale signals reaching the liquid crystal cells based on information concerning the typical dependence between the optical transparence of the liquid crystal cells and the voltage applied to them and as a function of the difference in potential between the gray-scale signals and the reference potential in such a manner that an at least approximately linear relationship arises between the optical transparence of the liquid crystal cells and the undistorted gray-scale signals.
The active matrix liquid crystal display of the present invention can thus also be operated within voltage ranges in which the transparence of the liquid crystal cells typically changes in a non-linear fashion as a function of the particular voltage applied to them without the occurrence of corruptions of the image rendering. This makes it advantageously possible to set optimum contrast ratios for specific viewing angle ranges via a largely unlimited selection of the voltage range and to better adjust the range within which the transparence of the liquid crystal cells is changed to display the gray-scale values to the background lighting.
The correction device can be connected to the column control unit, it distorting the gray-scale signals generated by the column control unit before they are output to the column electrodes. The signal distortion may be analog or digital depending on whether the gray-scale signal values are present in analog or digital form. The information concerning the typical dependency of the optical transparence of the liquid crystal cells on the applied voltage may be present in a memory as a characteristic curve or in the form of digital tabular values.
In an alternative embodiment of the active matrix liquid crystal display of the present invention, the correction device is connected to the row control unit, the row control unit changing the turn-on signals for the switches by controlling the turn-on times and turn-off times for the purpose of distorting the gray-scale signals transferred from the switches to the liquid crystal cells. In doing so, the integrating behavior of both the liquid crystal cells, which form individual capacitors, and of the human eye is used in that changing the relationship between the periodically successive turn-on and turn-off times of a liquid crystal cell activated with a specific gray-scale signal correspondingly changes the displayed or perceived gray-scale value.
Corresponding to a further development of the active matrix liquid crystal display of the present invention, it has an adjustment device for variably adjusting the difference in potential between the potential level of the gray-scale signals and the reference potential for at least a part of the columns. Changing the potential difference changes the viewing angle range within which the image displayed on the active matrix liquid crystal display is visible to the observer. Since the gray-scale values are distorted by the correction device as a function of the potential difference, its change does not result in a corruption of the image rendering. It is therefore possible in a like manner, as is known from the aforementioned U.S. Pat. No. 5,526,065, to mask out the displayed images in a vehicle in the viewing angle range of the driver while they are visible to the front-seat passenger; in contrast to the known methods, the image is rendered via the active matrix liquid crystal display which is by far better suited for use in vehicles than conventional screens due to, among other things, its smaller mounting depth. An additional possibility for use of the active matrix liquid crystal display of the present invention is the display of three-dimensional objects, different images of one and the same object being displayed in different viewing angle ranges which can be set.
In the simplest case, the adjustment device is designed for the variable adjustment of the reference potential.
As an alternative, the adjustment device can be designed for the variable adjustment of the potential level of the gray-scale signals, a variable offset voltage being superimposed, for example, on the analog gray-scale signals or a variable offset value being superimposed on the digital gray-scale signals.
To be able to display different images for different viewing angle ranges in an advantageous manner, according to the present invention the column control unit outputs the gray-scale signals of at least two different images interleaved chronologically in sequence to the column electrodes and the adjustment device sets different potential differences in sequence for the various images.
In an alternative embodiment of the active matrix liquid crystal display of the present invention, the column control unit outputs the gray-scale signals of at least two different images interleaved simultaneously column-by-column to the column electrodes and the adjustment device sets different potential differences at the column electrodes each assigned to different images.
The chronological interleaving and the spatial interleaving in the rendering of different images can also be combined. With the interleaved rendering of different images, it being possible, for example, to display traffic information to the driver in a vehicle and to present a video film to the front-seat passenger at the same time. In a like manner, it is possible to show different images (videos) via a single active matrix liquid crystal display to passengers sitting side-by-side in railroad cars or airplanes.
To further explain the invention in detail, reference will be made to the figures of the drawing in which: